Final answer:
In a reaction where 100.0 molecules of H₂ and 40.0 molecules of O₂ are combined, 80 molecules of H₂O can be produced because oxygen is the limiting reactant.
Step-by-step explanation:
If 100.0 molecules of H₂ and 40.0 molecules of O₂ react, the chemical reaction to consider is 2 H₂(g) + O₂(g) → 2 H₂O(g). This balanced chemical equation tells us that it takes two molecules of hydrogen gas to react with one molecule of oxygen gas to produce two molecules of water gas. Since we interpret the coefficients as molar amounts, we can also read them as the ratio of individual molecules.
According to the stoichiometry of the reaction, every 2 molecules of H₂ require 1 molecule of O₂ to form 2 molecules of H₂O. With 100.0 molecules of H₂ available, we would need 50.0 molecules of O₂ for the reaction to proceed completely. However, we only have 40.0 molecules of O₂, so oxygen is the limiting reactant. We can only react 40 molecules of O₂ with 80 molecules of H₂ to produce 80 molecules of H₂O.